In fuel cells, a fuel such as hydrogen or methanol is electrochemically oxidized so as to directly convert chemical energy of such fuel into electric energy, such that the energy can be extracted. In recent years, fuel cells have been attracting attention as clean electric energy supply sources. Particularly, solid polymer fuel cells using proton conductive membranes as electrolytes can achieve high power densities, and they can be operated at low temperatures. Thus, solid polymer fuel cells have been expected to serve as power supply sources for electric cars.
In the basic structure of such solid polymer fuel cell, a single cell is constituted with an electrolyte membrane sandwiched in contact with a pair of gas diffusion electrodes having catalyst layers, and current collectors are disposed both sides of the single cell. To the gas diffusion electrode (anode) on one side of the electrolyte membrane, a fuel such as hydrogen or methanol is supplied. Also, to the gas diffusion electrode (cathode) on the other side thereof, an oxidant such as oxygen or air is supplied. Then, by connecting an external load circuit to both gas diffusion electrodes, such solid polymer fuel cell can be activated. At such time, protons generated at the anode move to the cathode by passing through the electrolyte membrane so as to react with oxygen at the cathode, resulting in water generation. Here, the electrolyte membrane functions as a proton-transferring medium and a diaphragm between hydrogen gas and oxygen gas. Thus, a polymer electrolyte membrane for fuel cells is required to be excellent in terms of gas barrier performance, as well as to have the high proton conductivity, strength, and chemical stability.
Conventional so-called functional membranes have been problematic due to the fact that functional groups are distributed over such membranes in a random manner, and that when such membranes have a labyrinth or mesh structure, in which functional groups are contained, functional groups cannot be controlled in terms of spatial distributions or densities.
Specifically, in the case of a commercially available electrolyte membrane such as Nafion (trade name) or a solid polymer electrolyte membrane produced through radiation graft polymerization, hydrophilic cation exchange groups are uniformly distributed inside of a membrane, resulting in swelling of the membrane due to excessive moisture. As a result, interaction forces between molecules decline, so that excessive crossover of hydrogen or methanol through the membrane occurs. In addition, it has been attempted by Gore, Tokuyama Corp., etc. to fill a porous membrane with very high porosity, which has a series of holes in three dimensions, with ion exchange resins; however, the presence of ion exchange resins that are not involved in cation exchange results in excessive swelling of the membrane. Further, the porous substrate used is limited to a substrate comprising polytetrafluoroethylene or polyethylene, which can become porous. Furthermore, originally, such substrate lacks the gas barrier performance that is required for electrolyte membranes for fuel cells. Therefore, the characteristics of such solid polymer electrolyte membrane obtained have not been sufficient in the light of the required characteristics for fuel cells.